Twitter 'hijacked by Iranian hackers'

Popular online messaging service Twitter was left reeling overnight, after Iranian hackers appeared to break into the site and deface it.

The strike left the site completely unavailable for several hours in the early hours of Friday morning, with the site's estimated 30m users unable to access the service or send messages to each other online.

The incident took place some time around 6am in the UK, when the main Twitter page suddenly seemed to disappear – instead replaced with a stark black and red screen featuring an image of a flag.

The page, which carried a mixture of English and Farsi slogans, appeared to name the group behind the attack and offer a call to arms.

"This site has been hacked by the Iranian Cyber Army," said the message.

"The USA thinks they control and manage internet access, but they don't. We control and manage the internet with our power, so do not try to the incite Iranian people."

The site returned to normal functions around two hours later, with staff telling users that it had suffered from "unplanned downtime".

Although early reports suggested the site itself had been breached by attackers, it now seems that the strike was actually a crude form of assault known as a DNS hijack.

The DNS, or Domain Name System, is effectively a telephone directory of the internet – connecting the name of a website, such as twitter.com or theguardian.com, to the web servers that hold its contents.

In hijacking cases, computer criminals effectively redirect the traffic intended for a particular website, sending users to a page of their own choice rather than the planned destination.

Around two and half hours after the outage occurred, Twitter staff issued a short statement on the company's blog confirming the style of the attack it had suffered.

"Twitter's DNS records were temporarily compromised but have now been fixed," said the post. "We are looking into the underlying cause and will update with more information soon."

It is not the first time that Twitter has found itself subject to attention due to its links with Iranian activists.

During the uprising that followed the elections in June this year, in which incumbent president Mahmoud Ahmadinejad eventually triumphed, the US state department urged the site to remain online to allow more information about the protests to spread online.

Little is known, however, about the group who appeared to claim responsibility for hacking Twitter. But the nature of the messages they left appears somewhat confusing.

Though the text left by the hackers appeared to be anti-American, they also used the image of a green flag – the colour connected to the election protesters, and to Mir-Hossein Mousavi, the main challenger to President Ahmadinejad.